[sdiy] CV input circuit

[Corey K]

Thanks for the feedback everyone. I have seen Olivier's designs, and they
make perfect sense to me. Incidentally, those design are very similar as
what Eric Brombaugh used in his cheep mod module (
http://ebrombaugh.studionebula.com/synth/cheep2/index.html), which is one
of the other open designs I examined.

Anyway, I was just trying to get a better idea regarding the different
circuit designs currently being used. When I looked at the SMR circuit, it
wasn't immediately clear to me what was happening.

BTW, I think I figured out how to calculate the open circuit voltage on the
MORPHADC pin: I just consider the two voltage sources separately (i.e., the
5V source and then the op-amp) and use the principle of superposition to
combine them. As Olivier pointed out, the load impedance of this pin will
affect the actual voltage). As Roman said, the 5V source sets the mean
voltage, and the CV input is added to this.

On another note, it seems as though the dynamic range of the input is not
the same for all trim pot positions. E.g., then the pot is turned all the
way off (so that the 5V source becomes directly connected to R20), the open
circuit voltage output on the MORPHADC pin will be around 3.3V. Anything
added from the CV input (which is supposed to be in the range 0-10V) will
be clipped by the input protection diodes. This seems like a poor design
decision, but perhaps I'm wrong in my analysis of this circuit...


Thanks,
Corey




On Mon, May 14, 2018 at 12:40 PM, Tom Wiltshire <tom at electricdruid.net>
wrote:

> +1 agree. I’d do it Olivier’s way too, for the reasons given. One op-amp
> mixer per CV input isn’t a big deal and a reference voltage is not hard to
> rustle up either.
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
> On 14 May 2018, at 17:57, O Gillet <ol.gillet at gmail.com> wrote:
>
> >  guess I'm having a bit of difficulty figuring out exactly what the
> voltage will be at the MORPHADC and SPREADADC pins.
>
> For this you have to know the input impedance of the MORPHADC pin.
>
> I'm not a big fan of this kind of circuit. I prefer the way I do it (with
> a rail to rail op-amp in inverting configuration): the op-amp has a low
> output impedance, which is good for driving ADC inputs; the input impedance
> can be set to 100k precisely, which works nicely with some passive modules;
> you can do some low-pass filtering too. Costs an op-amp per input, and a
> voltage reference for your whole board.
>
> On Mon, May 14, 2018 at 6:25 PM Corey K <coreyker at gmail.com> wrote:
>
>> I've been studying a few open source circuit designs for digital eurorack
>> modules, and I've come across one that I'm having a little difficulty
>> analyzing. It is for the 4ms spectral multiband resonator (SMR). In
>> particular, I'm looking at circuits for the MORPHCV and SPREADCV inputs,
>> which are on sheet 4 of the schematic (left hand side):
>>
>> https://github.com/4ms/SMR/blob/master/hardware/smr-1.0.1-schematic.pdf
>>
>> I understand the basic parts of the circuit: the input passes through and
>> op-amp buffer (gain=1), and the output is connected to a 5v source via a
>> trim pot. The input to a microcontroller is clipped to an acceptable range
>> [0, 3.3V] with two diodes.
>>
>> I guess I'm having a bit of difficulty figuring out exactly what the
>> voltage will be at the MORPHADC and SPREADADC pins. I tried to do some
>> nodal analysis, but things weren't making sense to me.
>>
>> My intuition is that this is some type of "summing circuit" (the current
>> from the 5v source gets added to the current from the op-amp). If so, why
>> wouldn't the designer have used a traditional op-amp summing circuit? What
>> might be preferable about this configuration?
>>
>> Thanks,
>> Corey
>>
>>
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